BACKGROUND OF THE INVENTION
The present invention relates to an apparatus, such as a solid electrolyte fuel cell, incorporating compliant electrode/electrolyte structures that include opposing electrodes disposed on and electrically interconnected through a compliant electrolyte sheet, and a method of making such structures and apparatus.
The use of solid electrolyte materials for fuel cells and oxygen pumps has been the subject of a considerable amount of research for many years. The typical essential components of a solid oxide fuel cell (xe2x80x9cSOFCxe2x80x9d) include a dense, oxygen-ion-conducting electrolyte sandwiched between porous, conducting metal, cermet, or ceramic electrodes. Electrical current is generated in such cells by the oxidation, at the anode, of a fuel material, such as hydrogen, which reacts with oxygen ions conducted through the electrolyte from the cathode.
Practical power generation units will typically include multiple fuel cells of such configuration interconnected in series or parallel with electronically conductive ceramic, cermet, or metal interconnect materials. At the present time, the materials of choice for such devices include yttria-(Y2O3) stabilized zirconia (ZrO2) for the electrolyte, nickel-ZrO2 cermet for the anode material, strontium-doped lanthanum manganite (LaMnO3) for the cathode, and metals, especially Cr/Fe alloys and Ni alloys, intermetallics, and Sr or Ba doped LaCrO3, for interconnect structures. Alternative oxygen ion conductors are known. At sufficient temperatures (e.g., 600xc2x0 C. or above), zirconia electrolytes can exhibit good ionic conductivity but low electronic conductivity.
Several different designs for solid oxide fuel cells have been developed, including, for example, a supported tubular design, a segmented cell-in-series design, a monolithic design, and a flat plate design. All of these designs are documented in the literature, with one recent description in Minh, xe2x80x9cHigh-Temperature Fuel Cells Part 2: The Solid Oxide Cell,xe2x80x9d Chemtech., 21:120-126 (1991).
The tubular design comprises a closed-end porous zirconia tube exteriorly coated with electrode and electrolyte layers. The performance of this design is somewhat limited by the need to diffuse the oxidant through the porous tube. Westinghouse has numerous U.S. patents describing fuel cell elements that have a porous zirconia or lanthanum strontium manganite cathode support tube with a zirconia electrolyte membrane and a lanthanum chromate interconnect traversing the thickness of the zirconia electrolyte. The anode is coated onto the electrolyte to form a working fuel cell tri-layer, containing an electrolyte membrane, on top of an integral porous cathode support or porous cathode, on a porous zirconia support. Segmented designs proposed since the early 1960s (Minh et al., Science and Technology of Ceramic Fuel Cells, Elsevier, p. 255 (1995)), consist of cells arranged in a thin banded structure on a support, or as self-supporting structures as in the bell-and-spigot design.
A number of planar designs have been described which make use of free-standing electrolyte membranes. A cell is formed by applying single electrodes to each side of an electrolyte sheet to provide an electrode-electrolyte-electrode laminate. Typically these single cells are then stacked and connected in series to build voltage. Monolithic designs, which characteristically have a multi-celled or xe2x80x9choneycombxe2x80x9d type of structure, offer the advantages of high cell density and high oxygen conductivity. The cells are defined by combinations of corrugated sheets and flat sheets incorporating the various electrode, conductive interconnect, and electrolyte layers, with typical cell spacings of 1-2 mm for gas delivery channels.
U.S. Pat. No. 5,273,837 to Aitken et al. covers sintered electrolyte compositions in thin sheet form for thermal shock resistant fuel cells. It describes an improved method for making a compliant electrolyte structure wherein a precursor sheet, containing powdered ceramic and binder, is pre-sintered to provide a thin flexible sintered polycrystalline electrolyte sheet. Additional components of the fuel cell circuit are bonded onto that pre-sintered sheet including metal, ceramic, or cermet current conductors bonded directly to the sheet as also described in U.S. Pat. No. 5,089,455 to Ketcham et al. U.S. Pat. No. 5,273,837 to Aitken et al. shows a design where the cathodes and anodes of adjacent sheets of electrolyte face each other and where the cells are not connected with a thick interconnect/separator in the hot zone of the fuel cell manifold. These thin flexible sintered electrolyte-containing devices are superior due to the low ohmic loss through the thin electrolyte as well as to their flexibility and robustness in the sintered state.
Another approach to the construction of an electrochemical cell is disclosed in U.S. Pat. No. 5,190,834 Kendall. The electrode-electrolyte assembly in that patent comprises electrodes disposed on a composite electrolyte membrane formed of parallel striations or stripes of interconnect materials bonded to parallel bands of electrolyte material. Interconnects of lanthanum cobaltate or lanthanum chromite bonded to a yttria stabilized electrolyte are suggested. Unfortunately, the electrolyte/interconnect junctions in this design are sufficiently weak that a usefully compliant electrode/electrolyte structure cannot be obtained.
The internal circuit of the fuel cell circuit consists of the electrolyte, electrodes, and current conductors. The performance of a fuel cell, i.e., the current carrying capacity and hence the overall efficiency of the cell, is limited by its internal resistance, the maximum power for any power supply being given by Pmax=V2/4Rinternal. Internal resistance is the sum of several components including the electrode ohmic resistance, the electrolyte resistance, the electrode/electrolyte interfacial resistance to charge transfer reaction, and the current conductor resistance. The interfacial resistance to charge transfer depends mainly on the electrochemical behavior and physical and chemical nature of the electrode.
Precious or xe2x80x9cnoblexe2x80x9d metals such as gold, silver, platinum, palladium, rhodium etc. have been suggested as candidates for electrode materials in high temperature fuel cells, silver and its alloys, including silver-palladium, being amongst the best electrical conductors known. One disadvantage of silver as an electrode material, however, is its high volatility at temperatures over about 800xc2x0 C. Fuel cell operation at temperatures in the neighborhood of 700xc2x0 C. would significantly reduce metal volatilization, and should also allow the use of relatively inexpensive stainless steel components for the fabrication of manifold and other mechanical elements of the cells. Also, the volatility of precious metal electrode materials such as silver can be further reduced by introducing them in admixture with refractory ceramic fillers as cermet electrodes. Silver/yttria-doped zirconia cermet cathodes, for example, are advantageous due to high electronic conductivity and good catalytic properties, and can be made economically in thin film form by continuous magnetron sputtering.
The present invention is directed to providing an improved fuel cell construction, applicable to any of a variety of planar fuel cell designs, which avoids many of the difficulties of fuel cell manufacture while providing a cell of improved physical, thermal, and electrical properties.
In an important aspect, the present invention is based on a thin, compliant electrode/electrolyte structure for a solid oxide fuel cell that offers improved cell design flexibility while retaining high strength, mechanical integrity, and resistance to thermal degradation from temperature cycling and thermal shock. The electrode/electrolyte structure of the invention comprises a thin flexible solid oxide electrolyte sheet incorporating a plurality of positive air and negative fuel electrodes bonded to opposing sides of the sheet. Thus the electrodes do not form continuous layers on the electrolyte sheets, but instead define multiple discrete regions, typical bands or other segments. The segments are then electrically interconnected in series, parallel, or a combination thereof by means of electrical conductors in contact therewith that extend through vias in the electrolyte sheet. The vias are filled with electronically conductive materials, preferably of lower electrical resistance than the electrodes.
Electronic conductor materials suitable for forming the electrical interconnections through the vias are may be metallic, ceramic, or cermet electronic conductors. Metallic conductors are preferred for their higher conductivity and better sintering characteristics, examples of suitable conductors including precious or semi-precious metals or their alloys. For the purpose of the present description precious metals include metals selected from the group consisting of silver, gold, platinum, palladium and rhodium.
In another aspect, the invention resides in a solid oxide fuel cell incorporating one or more compliant electrode/electrolyte structures such as above described. The electrodes in these cells are attached to opposing sides of the electrolyte structure in configurations effective to provide multiple power-generating segments within the fuel cell that can be connected in various groupings to provide electrical energy at predetermined levels of voltage or current. Where multiple electrical series connections between the electrodes are employed, fuel cells offering higher voltage outputs for specific applications are conveniently provided.
The electrode/electrolyte structures supporting the multiple electrode segments may easily be combined to provide fuel cell stacks wherein the electrode/electrolyte structures are disposed as layers. The layering is desirably carried out so that the fuel or air electrodes of adjacent structures face each other in arrangements forming reservoirs for air or fuel between the facing layers, these reservoirs conveniently being supplied by fuel or air manifolds connecting therewith. The electrolyte structures are effective to maintain air-fuel reservoir separation without the use of additional gas separators. Conventional interconnecting stack elements may be used to connect multiple electrode/electrolyte structures; electrode interconnections are provided by vias in the electrolyte structure.
Still another aspect of the present invention relates to a method of making a compliant electrolyte/electrode element for a fuel cell apparatus. In accordance with that method a compliant ceramic electrolyte sheet is selected and electrode layers comprising multiple electrode segments separated by gaps therebetween are applied to opposing sides of the sheet. Cathode segments are deposited on one side of the sheet and anode segments are deposited in opposition to the cathode segments on the other side of the sheet to provide the basic components of a series of electrochemical cells.
A plurality of vias is formed in the sheet, these typically being placed at sheet locations not covered by the anode and cathode segments. Electrically conducting materials are then applied to the sheet to fill the vias and to provide electrical connections between the cathodes and the anodes. Each electrical conductor traverses one or more of the vias and is arranged to be in contact with a cathode segment on one side of the sheet and an opposing anode segment on another side of the sheet.
Appropriate ceramic or cermet components may be employed to form the electrodes of the present invention, these being selected to minimize electrode interface resistance and improve electrode durability. Electrodes so comprised can display exceptionally low ohmic and interfacial resistance for both the air-side (cathode) and fuel side (anode) electrodes. Moreover, some silver-based electrode compositions can impart excellent tolerance towards leakage of fuel into the air chamber or air into the fuel chamber. This can be advantageous since complete avoidance of leakage, whether from pinholes through the electrolyte or egress through seals, is difficult to achieve.
Fuel cell stack designs incorporating thin, compliant, self-supporting electrolyte sheets supporting thin electrode segments as above described exhibit high active element flexibility and compliance, and are therefore highly thermal shock tolerant. Further, combining the compliant electrolyte sheets with symmetric electrodes having similar thermal expansion behavior can improve stress field symmetry in the sheets, giving rise to a flat composite body well suited for use in fuel cell stacks.